The present invention relates to semi-insulating gallium arsenide single crystal material and methods of fabrication which produce a crystal structure exhibiting a low dislocation density.
While silicon remains the primary semiconductor substrate material for commercial integrated circuit technology, gallium arsenide substrates exhibit electrical characteristics which permit improved integrated circuit device capability. The areas of gallium arsenide development have centered on improvements in crystal purity, dislocation density, and uniformity which are necessary for widespread, reasonable cost usage of gallium arsenide in commercial integrated circuit fabrication.
Gallium arsenide single crystals have been fabricated using conventional liquid encapsulation Czochralski growth technique. The gallium and arsenic are disposed in a refractory crucible and thermally melted, with the melt being covered with liquid boric oxide with a high inert gas pressure maintained in the system. The liquid boric oxide and high pressure served to impede volatilizaion of the arsenic from the melt. Such crystal growth techniques are shown in Great Britain Patent Application No. 2,108,404 published May 18, 1983, and in the article entitled "Preparation and Properties of Bulk In.sub.1-x Ga.sub.x As Alloys," by J. Wagner, from the Journal of Electrochemical Society Solid State Science, September 1970, pp 1194 through 1196. These earlier works teach the incorporation of dopants such as indium and boron to improve crystal structure and particularly to minimize dislocation density. The fabrication of gallium arsenide single crystal by liquid encapsulated Czochralski growth processes is associated with severe thermal stresses which tend to produce high dislocation densities, typically in the 10.sup.4 to 10.sup.5 cm.sup.-2 range, which limit the use of gallium arsenide crystals as subtrates for field effect transistor fabrication. The effects of high dislocation defects in gallium arsenide substrates upon the field effect transistors formed therein are reported by several recent articles, "Direct Observation of Dislocation Effects on Threshold Voltage of a GaAs Field Effect Transistor," by S. Miyazawa et al., from Applied Physics Letters, Nov. 1, 1983, "Leakage Current I.sub.L Variation Correlated with Dislocation Density in Undoped, Semi-Insulating LEC-GaAs," by S. Miyazawa, published in Japanese Journal of Applied Physics, Vol. 21, No. 9, September 1982, pp. L542-L544, "Correlation between Disclocation Distribution and FET Performances Observed in Low Cr Doped LEC GaAs," by Y. Nanishi, published in the Japanese Journal of Applied Physics, Vol. 22, No. 1, January 1983, pp L54-L56.
In prior art techniques for growing gallium arsenide it has been found difficult to maintain the desired stoichiometric ratio which provides good electrical characteristics, and the yield of crystals with the desirable characteristics is low.
It is also well known that to ensure semi-insulating electrical behavior in gallium arsenide single crystal material, the ratio of gallium to arsenic in the melt must be carefully controlled tp achieve large diameter crystals exhibiting uniform N-type high resistivity over the full crystal length with the most desirable composition corresponding to a slightly arsenic rich melt.
The desired electrical characteristics of semi-insulating gallium arsenide useful for integrated circuit applications exhibits a high resistivity as well as high mobility. This material is typically used as a substrate with silicon ion-implanted as an active surface layer permitting fabrication of semiconductive devices.
As is well understood, the grown crystal is sliced into wafers which are then processed to form the semiconductive devices. Such gallium arsenide crystals and wafers exhibit a lattice structure which is subject to fracturing during fabrication handling of the wafers. It is desired to strengthen this lattice structure.
In other applications gallium arsenide is used in opto-electronic devices as a substrate with thin ternary films of, for example, gallium-aluminum-arsenide epitaxially grown on such substrate. The lattice parameters of the substrate and the deposited film are generally mismatched, giving rise to misfit dislocations at the interface of the substrate and the film.